Interesting. If it's the front main seal, have them recheck the oil level carefully before giving you the car back. Mercedes dealers are notorious for overfilling the crankcase (I have no idea why) and this causes oil cavitation on the main seal, slowly (or quickly) destroying it.
The engine has no dipstick

but the dealer has one.
I finally stopped taking my car to any dealer after a series of main seal replacements thanks to overfilling. Three different dealers, all of them overfilled the crankcase. On the MB forum, I found out this behaviour is widespread. No one has a clue of why, as it actually costs them money in additional oil.